A molecular force field refers to a field of force acting on each atom that comprises a molecule. Thus, if a molecular force field is known, the geometrical structure, electronic properties, physical properties and reactivity of a molecule can be simulated. For example, a simulation can be carried out to determine the manner in which a candidate substance of a new drug reacts with a known protein comprising the human body or a virus.
This type of simulation is referred to as a molecular orbital method calculation in the case the force field used in the calculation is represented by a function based on quantum mechanics. On the other hand, this type of simulation is referred to as a molecular mechanics calculation or molecular force field method in the case the force field used in the calculation is represented by a function based on classical physics.
In a molecular orbital method calculation, the molecular orbital function is determined by solving a Schrödinger equation. Thus, a highly precise solution is obtained that takes into consideration quantum effects. However, the calculation becomes difficult as the number of atoms that compose the molecule increases since the number of calculations rapidly increases.
Consequently, molecular force field calculations were created to simulate the structures of molecules having a large number of constituent atoms in the manner of organic compounds. The functions used in molecular force field calculations are classical potential functions, primary examples of which include bond stretching energy obtained by hypothesizing that elastic force in the manner of a spring acts between atoms, angle bending energy of a bond angle similarly based on the hypothesis that elastic force acts between bonds of the same atom, torsion energy based on the hypothesis that elastic force acts between dihedral angles, van der Walls non-bonding interaction energy, and electrostatic interaction energy between ions (“Viewing the Shapes of Molecules by Computer”, Shokabo Publishing Co., Ltd., 2005, p. 31.).
An example of a simulation in which the molecule force field calculation demonstrates its effectiveness is conformational analysis. Even though an organic compound may have the same molecular formula, it may have a plurality of possible three-dimensional structures. In such cases, use of a molecular force field calculation makes it possible to determine which of the structures are stable (conformational analysis).
On the other hand, molecular dynamics calculations, which analyze interactions between molecules in accordance with Newton's equation of motion by hypothesizing that forces of classical dynamics act between molecules, are effective in predicting reactivity between molecules as well as the equilibrium state and physical properties of molecule groups. Since molecule dynamics calculations analyze molecular interactions and the passage of time, the number of calculations is far greater than that of molecular mechanics calculations. Thus, classical potential functions, for which calculations are easier, are used as functions for representing molecular force fields in the same manner as the molecular force field method. However, the objects of molecular dynamics calculations are molecule groups in which interactions acting between molecules are comprised only of non-bonding interactions. Namely, molecular dynamics calculations only take into consideration electrostatic interaction energy and van der Waals non-bonding interaction energy.
Molecular orbital method calculations have been able to be applied even to considerably large molecules due to the significant improvement in the processing speeds of computers. Thus, the number of molecular structure calculations forced to rely on molecular force field calculations is decreasing. However, even if using current high-speed computers, it is still difficult to analyze molecular interactions and passage of time by quantum mechanics. Thus, molecular dynamics calculations have not declined in importance even at present.
On the contrary, molecular simulations using molecular dynamics calculations are actively used in the fields of biochemistry by utilizing high-speed computers. In order to develop a new drug, it is necessary to comprehensively produce new drug candidate substances and then test reactions between the drug and proteins it is to act on (consisting mainly of reactions resulting from clone interactions between atoms forming molecules). However, if it were possible to predict the reaction between a new drug candidate substance and a target protein by molecular dynamics calculations, it would be possible to considerably reduce development costs and shorten development time. Thus, molecular dynamics calculations are extremely important in the development of new drugs.
Furthermore, in the case of talking about molecular force fields, this usually refers to the classical potential used in molecular force field calculations or molecular dynamics calculations. Thus, a molecular force field (or force field) hereinafter refers to the classical potential used in molecular force field calculations or molecular dynamics calculations.    Non-Patent Document 1: “Viewing the Shapes of Molecules by Computer”, Shokabo Publishing Co., Ltd., 2005, p. 31.    Non-Patent Document 2: “Fast, Efficient Generation of High-Quality Atomic Charge, AM1-BCC Model II, Parameterization and Validation”, Journal of Computational Chemistry, Vol. 23, p. 1623-1641, 2002.